Optical interference filters rely on principles of interference that modify reflected intensities of light incident on a surface. A familiar example of interference is the colors created when light reflects from a thin layer of oil floating on water. Briefly stated, by modifying the interface of a substance and its environment with a third material, reflectivity of the substance can be significantly altered. This principle is used in the fabrication of optical interference filters. These filters can be used as one of, or as the main filtering element in optical add/drop multiplexers employed in optical communication systems to select one or more channels from a transmission signal.
In its most simple form, an optical interference filter includes a cavity which is comprised of two partial reflectors separated by a spacer. Each partial reflector, also referred to as a quarter-wave stack, is typically constructed by depositing alternating layers of high and low refractive index dielectric materials upon a substrate where each layer has an optical thickness (defined as: physical thickness.times.refractive index) of a quarter wave (.lambda./4) at the desired wavelength of the filter. The spacer is typically a half-wave (or multiple half-wave) layer. An interference filter has an associated transmission characteristic which is a function of the reflectance of the layers of high and low index materials associated with the stack.
In many applications, optical interference filters are constructed using multiple cavities. Typically, cavities are deposited on top of other cavities, with a quarter-wave layer of low index material therebetween. Multicavity filters produce transmission spectra that are preferred in optical communication systems where sharp slopes and square passbands are needed to select one or more optical channels. The larger the number of cavities employed, the steeper the slope of the transmission bandwidth associated with a particular filter. The transmission bandwidth of a multicavity filter is wider as compared with the transmission bandwidth associated with a single cavity filter.
FIG. 1 illustrates an exemplary transmission spectrum (normalized to 1.55 .mu.m) for a quarter-wave stack having a plurality of high/low refractive index dielectric layers. The stack is tuned to reject wavelengths in the 1.5.mu.m range and exhibits ripple sidelobes referenced at 5.
FIG. 2 is an exemplary transmission spectrum (normalized to 1.55 .mu.m) for a single cavity optical interference filter utilizing a pair of stacks each having the transmission spectrum shown in FIG. 1. As can be seen in FIG. 2 the transmission response is acceptable at .lambda./.lambda..sub.0 =1.0 which corresponds to 1.55 .mu.m (.lambda./.lambda..sub.0 =1.55 .mu.m/1.55 .mu.m). However, the response at 0.845 which corresponds to approximately 1.31 .mu.m (.lambda./.lambda..sub.0 =1.31 .mu.m/1.55 .mu.m) falls on the sideband and/or within the ripple band of the transmission spectrum, thereby making transmission of a particular wavelength in this range unreliable. More specifically, the single cavity interference filter produces high transmittance at wavelengths referenced at 10, but also produces relatively low transmittance as referenced at 15. Thus, transmission at wavelengths in the 1.5 .mu.m range may be reliable while transmission for wavelengths within the ripple band or sideband slope are subject to variations in the transmission characteristic. This is also true for wavelengths in the 1.6 .mu.m range (.lambda./.lambda..sub.0 =1.62 .mu.m/1.55 .mu.m). FIG. 2 demonstrates that interference filters typically provide a single reliable passband.
As noted above, optical systems can utilize one or more interference filters to select particular channels from a transmission signal. For example, a first filter may be used to select a pay-load channel associated with voice and/or data transmission in the 1.5 .mu.m range and a second filter is used to select a service channel in the 1.3 .mu.m or 1.6 .mu.m range which carries system level and/or network monitoring information. The use of two separate filters, however, has several disadvantages. First, it increases overall system cost since it requires the manufacture and installation of two individual components. Secondly, optical networks typically have a predetermined loss budget, if exceeded, can compromise signal integrity. Each component, in this case an optical filter, contributes some loss to the overall network. By using two separate filters to select a payload channel and a service channel, each filter negatively impacts a network's loss budget.
Thus, there is a need for a filtering element used with optical communication systems capable of selecting a first and a second optical passbands. There is a further need to provide such a filtering element which reliably selects at least one wavelength corresponding to a payload channel as well as a wavelength corresponding to a service channel within an optical network.